A known scanning electron microscope (SEM) comprises an electron optical column, a voltage source, an Everhart-Thornley detector and a sample holder, the column being operable to direct a beam of primary electrons at a sample on the sample holder to cause secondary electrons to be released from the sample, the voltage source being operable to establish in the vicinity of the sample an electric field that has a component that draws the secondary electrons towards the detector, and the detector being operable to detect some of the secondary electrons.
A key feature of the SEM is its resolution. In general the smallest spot size (spot size is the diameter of the beam of primary electrons at the surface of the sample) is obtainable at short working distances. (Working distance is the distance from the sample to an end of an objective lens of the column nearer to the sample) because the aberration coefficients decrease with decreasing working distance provided that the optimum semi angle can be maintained. (The semi angle is an angle included by a tip of a cone formed as the beam of primary electrons is focused at the surface of the sample). The SEM is therefore typically used at short working distances.
However, the number of secondary electrons that are detected by the detector also decreases at short working distances because the secondary electrons are shielded from the electric field by the sample holder and a pole piece of the objective lens of the column.
As most of the secondary electrons have kinetic energies from only 2 eV to 5 eV a strong magnetic field of the objective lens of the column forces most of the secondary electrons to spiral up the column, especially at higher beam energies, or to collide with the sample, sample holder or column, resulting in a further reduction in the number of secondary electrons detected by the detector.